The present invention relates to a thrust balance device. More specifically, the present invention relates to a thrust balance device significantly improving thrust balance in a device such as a canned motor pump.
Conventional canned motor pumps include an impeller mounted on a rotating shaft. In this type of canned motor pump, a fluid is suctioned through a suction opening which faces the rotating shaft. Centrifugal force from the impeller causes discharge of the suctioned fluid from radial discharged openings. Since the suction opening is oriented toward the rotation shaft, a force is applied on the impeller in the direction of thrust. Thus, in primitive canned motor pumps, the impeller is pushed toward an inner wall of the chamber holding the impeller. This pushing force interferes with the rotation of the impeller. As a result, almost all recent canned motor pumps are equipped with a thrust balance mechanism.
Suction of the fluid generates a pressure in the direction of thrust. A thrust balance mechanism prevents obstruction of the rotation of the impeller caused by the pressure of the suctioned fluid. Generally, thrust balance mechanisms include:
(1) a fixed orifice wherein a ring-shaped cylinder is formed on a rear surface of an impeller having a balance hole. The ring-shaped cylinder is inserted into a cavity which has a cylindrical inner perimeter surface disposed on a casing. The result is a gap formed between the outer perimeter surface of the cylinder and the cylindrical inner perimeter surface of the cavity; PA1 (2) a thrust balance chamber which is formed from the following: a bottom surface of the cylinder; an inner perimeter surface of the cylinder; a surface of a first projection projected from the casing toward an inner space of the cylinder, this first surface facing the bottom surface and being separated from the bottom surface by a prescribed gap; and an outer perimeter surface of a ring-shaped second projection, surrounding the rotor, projecting further than the first projection; PA1 (3) a variable orifice which is formed from an end surface of the second projection facing the rear surface of the impeller and the rear surface of the impeller.
In the conventional thrust balance mechanism, the centrifugal force from the rotation of the impeller causes fluid to be discharged radially. A portion of the fluid discharged in the centrifugal direction flows into the thrust balance chamber via the fixed orifice. The fluid which enters the thrust balance chamber flows out from the thrust balance chamber through the variable orifice. The fluid exiting the thrust balance chamber passes through the balance hole and combines with the discharged fluid.
If the pressure of the suctioned and discharged fluid increases, a pressure in the direction of thrust is applied to the impeller. This pressure causes the back surface of the impeller to approach the casing surface facing the back surface of the impeller. However, pressure from the fluid also increases the flow rate, resulting in higher fluid pressure within the thrust balance chamber. The increase in fluid pressure in the thrust balance chamber causes a pressure to be applied on the impeller from the casing facing the rear surface of the impeller, pushing it away. This pressure is sometimes referred to as independent pressure. Fluid pressure within the thrust chamber causes the impeller to move against the pressure from the fluid being suctioned and discharged.
The gap in the variable orifice increases when the impeller is displaced away from the casing facing its rear surface, i.e., when the impeller is shifted so that it moves away from the casing surface facing the rear surface of the impeller. This movement causes high-pressure fluid to suddenly flow from the variable orifice. As a result, fluid pressure within the thrust balance chamber drops. The pressure in the thrust direction applied to the impeller from the fluid being suctioned and discharged becomes greater than the fluid pressure within the thrust balance chamber. The pressure in the thrust direction causes the impeller to shift toward the casing surface facing the rear surface of the impeller.
As described above, in order to balance the pressure within the thrust balance chamber and the pressure from the fluid being suctioned and discharged, the impeller changes its position based on the gap in the fixed orifice, the gap in the variable orifice, as well as the volume of the thrust balance chamber. The change of position of the impeller maintains balance for the rotor along the thrust direction.
However, with thrust balance chamber in conventional thrust balance devices, the rear surface of the impeller is a rotation surface, while the casing surface facing the impeller is a fixed surface. Thus, fluid flowing into the thrust balance chamber receives an angular momentum energy from the impeller rotation. Additionally, fluid flowing into the thrust balance chamber rotates together with the impeller. As a result, a very high flow-path resistance is generated in the thrust balance chamber by the fluid rotating with the impeller.
The flow-path resistance of the fluid interposed between the rotation surface and the fixed surface is proportional to the square of the peripheral speed of the fluid rotating with the rotation surface. Thus, a large amount of fluid is present in the gap between the fixed surface and the rotation surface, especially in high-speed pumps where the impeller rotation speed is very high. Furthermore, the flow-path resistance of fluid in the thrust balance chamber becomes large in large pumps where the peripheral speed of the rotating fluid is high, preventing the thrust balance from being maintained appropriately.
In order to overcome this problem, bypass structures known as pressure-equalizing holes or pressure-decreasing holes have been conventionally formed in the fixed surface of the thrust balance chamber. However, these pressure-equalizing holes have been unable to lower flow-path resistance and maintain thrust balance. While forming this kind of bypass may be able to increase the independent pressure, this kind of bypass cannot significantly reduce the angular momentum of the fluid inside the thrust chamber.